Wireless Local Area Network (WLAN) technology has been developed as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. IEEE 802.11a and 802.11b use an unlicensed band at 2.5 GHz or 5 GHz. IEEE 802.11b provides a transmission rate of 11 Mbps and IEEE 802.11a provides a transmission rate of 54 Mbps. IEEE 802.11g applies Orthogonal Frequency Division Multiplexing (OFDM) at 2.4 GHz to provide a transmission rate of 54 Mbps. IEEE 802.11n applies Multiple Input Multiple Output (MIMO)-OFDM to provide a transmission rate of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth up to 40 MHz and, in this case, provides a transmission rate of 600 Mbps.
Currently, IEEE 802.11af standard that specifies operation of an unlicensed device in a TV White Space (TVWS) band is being developed.
A TVWS includes Very High Frequency (VHF) bands (54 to 60, 76 to 88, and 174 to 216 MHz) and Ultra High Frequency (UHF) bands (470 to 698 MHz) allocated for TV broadcast and refers to frequency bands in which use of unlicensed devices is permitted under the condition that it does not hinder communication of licensed devices (a TV and a wireless microphone) operating at the corresponding frequency bands.
All unlicensed devices are permitted to operate in frequency bands of 512 to 608 MHz and 614 to 698 MHz except in a few special cases. However, frequency bands of 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz, and 470 to 512 MHz are used only for communication between fixed devices. The fixed devices refer to devices which perform transmission only in a determined location. In the following description, a white space band includes the above-described TVWS but is not limited thereto.
An unlicensed device which desires to use the white space band should provide a protection function for a licensed device. Accordingly, the unlicensed device must confirm whether the licensed device occupies the corresponding band before starting signal transmission in the white space band.
To this end, the unlicensed device must obtain information about an available channel list (i.e. a set of available channels) in a corresponding area by accessing a Geo-location Database (GDB) through the Internet or a dedicated network. The GDB is a database for storing and managing information about licensed devices registered thereto and information about channel use which dynamically varies according to geographic location and channel use time of the licensed device. To solve a coexistence problem between unlicensed devices using the white space, a signaling protocol, such as a common beacon frame, and a spectrum sensing mechanism may be used.
In an IEEE 802.11 system, a TVWS terminal may refer to an unlicensed device operating using an IEEE 802.11 Medium Access Control (MAC) layer and a Physical (PHY) layer. In this document, a station (STA) refers to a TVWS terminal operating in a TVWS spectrum unless otherwise mentioned.
The STA should provide a function for protecting an incumbent user or a primary user that is permitted primary access, including licensed users (TV users and radio microphones). That is, if the incumbent user is using the TVWS, the STA must stop using a corresponding channel. Therefore, the STA must detect channels available to the unlicensed devices (i.e. channels which are not used by licensed devices) to operate in available channels.
A method for the STA to detect available channels includes a method for performing a spectrum sensing mechanism and a method for detecting a TV channel schedule by accessing the GDB. The spectrum sensing mechanism may use an energy detection scheme (if strength of a received signal is greater than a given value, it is determined that an incumbent user is using the TVWS) and a feature detection scheme (if a digital TV preamble is detected, it is determined that an incumbent use is using the TVWS). Next, the STA should access the GDB and acquire GDB information based on location information thereof to discern whether channels of a licensed device are being used in a corresponding location. Access to the GDB and information acquisition should be performed a prescribed number of times sufficient to protect the licensed device.
If it is determined through spectrum sensing or GDB that an incumbent user is using a channel immediately adjacent to a currently used channel, a terminal (or STA) and an Access Point (AP) can protect the incumbent user by a scheme of lowering a transmit power.